Quenching Process and Apparatus for Practicing Said Process

ABSTRACT

A process for quenching heat treated metal parts using a liquid quenchant and high pressure is disclosed. In general, the process includes the steps of providing a load of heat treated metal parts in a pressure vessel wherein the load is at an elevated temperature after being heat treated. In a subsequent step, a liquid quenchant is injected into the pressure vessel such that a vapor of the liquid quenchant forms rapidly in the pressure vessel and cools the metal parts. The step of injecting the liquid quenchant into the pressure vessel is continued for a time sufficient to establish a desired peak vapor pressure in the pressure vessel. An apparatus for carrying out the disclosed process is also described.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/468,267, filed Mar. 28, 2011, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for quenching heat treated metallicwork pieces and to an apparatus for carrying out the method.

2. Description of the Related Art

In some of the known heat treatment systems, a high pressure gas quenchsubsystem is used to rapidly cool the metal work pieces from the heattreatment temperature. As shown in FIG. 1, the quenching subsystemincludes an accumulator tank 1 that stores a large volume of thequenching gas at a high pressure. When the accumulator tank empties intothe furnace or a standalone quenching chamber 2, the gas pressure in thefurnace or the quench chamber, as the case may be, rises quickly to thedesired quenching level.

In the case where the final quench pressure is high, e.g., on the orderof about 20-30 bar, for example, many large accumulator tanks would berequired, each storing gas at a pressure much higher than the finalquenching pressure. Such tanks are expensive and take up a lot of spacein the processing facility. The rapid filling of the furnace requires alarge pipe and valve size to allow the furnace to reach the final quenchpressure in a short time. In order to pressurize the large accumulatortanks to the required high pressures, a compressor system or very highpressure gas delivery system is sometimes employed. Both of thosesystems require additional energy to fill the tanks That energyultimately is wasted because it does not convert into useful energy inthe furnace quenching process.

The main problems the invention is meant to address are summarized asfollows.

1) Physical space used by high pressure backfill tank(s).

2) The compressor systems that charge these tanks to high pressures (upto 30 bar or more) have periodic maintenance issues with wear parts andalso add unwanted energy into the process of furnace quenching.

3) If a compressor system is not used, the end user of the furnaceequipment would have to change the bulk gas storage system in thefacility and the high pressure gas delivery line from what would betypically a 10 bar or an 18 bar gas delivery system to at least a 30 bargas delivery system.

4) Typically gas is kept in a liquid state in bulk storage systems. Ittakes energy to change the gas into a liquid form, energy that the enduser already paid for when they bought the liquid gas. If the liquid gasis used downstream of the bulk storage system, it commonly goes througha vaporizer to turn it back into a gaseous state before delivery. Theconversion of liquid gas to the gaseous state gives up stored energy bycooling the vaporizer. This energy is wasted and is not useful in thefurnace quenching process.

SUMMARY OF THE INVENTION

This invention provides a process and associated apparatus to deliver aliquid, a liquefied quenching gas or vapor directly into a furnacechamber such that the liquid, liquefied gas, or vapor converts to afully gaseous state thereby rapidly increasing the pressure inside thechamber.

The process and apparatus according to this invention eliminate the needfor large high pressure gas storage tanks The conversion of liquefiedgas to the gaseous state inside the furnace chamber utilizes the energystored in the liquefied gas and eliminates the need for compressors orother high pressure gas delivery systems.

In accordance with a first aspect of the present invention there isprovided a method for rapidly cooling a load of heat treated metal partsfrom an elevated temperature. The method includes the steps of injectinga pressurized liquid quenchant into a pressure vessel containing a loadof heat treated metal parts such that a vapor of the liquid quenchantforms rapidly and cools the metal parts and continuing to inject thepressurized liquid quenchant for a time sufficient to establish adesired peak vapor pressure in the pressure vessel. Preferably theliquid quenchant is readily vaporizable at temperatures and pressuresutilized for the heat treatment of metal work pieces.

In a preferred embodiment of the process the pressurized liquidquenchant is injected for a time sufficient to establish a vaporpressure in the pressure vessel of about 5 to 100 bar.

In another preferred embodiment the quenchant vapor is circulated in thepressure vessel at high velocity while the liquid quenchant is injectedinto the pressure vessel such that the quenchant vapor penetratesthrough the load of metal parts.

In another preferred embodiment the injecting step includes the step ofspraying the liquid quenchant in a preselected direction in the pressurevessel.

In further preferred process, the injecting step includes providing theliquid quenchant at an initial pressure prior to the start of theinjecting step that is higher than the desired peak vapor pressure inthe pressure vessel. Preferably the initial pressure of the liquidquenchant is higher than the quenchant vapor pressure in the pressurevessel by at least about 3 bar.

Preferably, the method comprises the step of continuously raising thepressure of the liquid quenchant during the injecting step such that theliquid quenchant pressure is always higher than the instantaneousquenchant vapor pressure in the pressure vessel.

Preferably the process includes the step of continuously raising thepressure of the liquid quenchant during the injecting step such that theliquid quenchant pressure is about 3 to 5 bar higher than theinstantaneous vapor pressure in the pressure vessel.

Preferably the injecting step is stopped once the desired peak vaporpressure in the pressure vessel is reached.

In another preferred embodiment the steps of maintaining the peakquenchant vapor pressure in the pressure vessel and continuing tocirculate the quenchant vapor are carried out for a time sufficient tolower the temperature of the metal parts to a temperature lower than theelevated temperature of the metal parts.

Preferably the process includes the step of continuing the injectingstep for a period of time after the peak vapor pressure in the pressurevessel is reached.

Preferably the peak vapor pressure in the pressure vessel is maintainedat the desired level by exhausting a portion of the quenchant vapor fromthe pressure vessel.

Preferably the peak vapor pressure in the pressure vessel is maintainedat the desired level by injecting additional liquid quenchant into thepressure vessel.

A further preferred embodiment includes the step of reducing thequenchant vapor pressure in the pressure vessel to a lower pressure whenthe load of metal parts reaches the first lower temperature.

Preferably the method includes the step of holding the quenchant vaporpressure in the pressure vessel at the lower pressure until the load ofmetal parts reaches a selected final temperature.

In a still further preferred embodiment the circulating step includescirculating the quenchant vapor through a heat exchanger and circulatinga heat absorbing fluid in the heat exchanger to absorb heat from thequenchant vapor.

In a still further embodiment the injecting step is carried out with aflow rate that is effective to raise the vapor pressure in the pressurevessel to the desired peak vapor pressure within about 2 to about 60seconds from the start of the injecting step.

In one embodiment, the process according to the invention uses aliquefied gas as the quenchant. In a particularly preferred embodimentthe liquid quenchant is selected from the group consisting of liquefiednitrogen, liquefied helium, liquefied argon, liquefied air, a liquefiedhydrocarbon gas, liquefied carbon dioxide, and a combination thereof. Inanother embodiment, a liquid quenchant such as water or an aqueousquenchant solution can be used to provide a high pressure steam quench.In a further embodiment, the process according to this invention iscarried out with oil as the liquid quenchant.

In accordance with a second aspect of this invention, there is providedan apparatus for rapidly cooling a work load of heat treated metalparts. An apparatus according to the invention includes a pressurevessel having an internal chamber for holding a work load of heattreated metal parts. The apparatus also includes a liquid quenchantsupply vessel adapted to contain a liquid quenchant at a first pressureand a quenchant conducting means for conducting the liquid quenchantfrom the supply vessel to the internal chamber of the pressure vessel.The apparatus further includes a pressure control means operativelyconnected to the pressure vessel and the quenchant conducting means formaintaining the liquid quenchant conducted to the pressure vessel at anelevated pressure differential sufficient to establish a desired peakvapor pressure in the internal chamber of the pressure vessel.

Preferably the pressure control means is adapted for controlling theflow rate of the liquid quenchant from the supply vessel to the internalchamber of the pressure vessel.

Preferably the quenchant conducting means comprises means for increasingthe pressure of the liquid quenchant conducted to the pressure vesselwhich may be embodied as a liquid pump or a source of pressurized gas.

In another preferred embodiment the quenchant conducting means includesa storage tank adapted for concurrently holding liquid and vapor phasesand means for increasing the vapor pressure inside the storage tank.

In a still further preferred embodiment the means for spraying theliquid quenchant comprises at least one spray nozzle mounted in thepressure vessel and connected to the means for conducting the liquidquenchant.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary of the invention as well as the following detaileddescription of the invention will be better understood when read inconjunction with the drawings, wherein:

FIG. 1 is a schematic diagram of a known system for supplying aquenching gas at high pressure to a pressure vessel chamber or quenchingchamber;

FIG. 2 is a schematic diagram of an embodiment of a high pressurequenching system in accordance with the present invention;

FIG. 3 is a graphical diagram of a gas quenching cycle in accordancewith the present invention; and

FIG. 4 is a graphical diagram of a second gas quenching cycle inaccordance with the present invention

DETAILED DESCRIPTION

Referring now to the drawing and in particular to FIG. 2, there is shownan embodiment of a high pressure gas quenching system 10 in accordancewith the present invention. The system 10 is configured for use with aheat treating furnace 12 that is equipped for high pressure gasquenching. Alternatively, the system 10 can be used with a stand-alonehigh pressure quenching chamber of the type to which a load ofheat-treated parts is moved for quenching. The system 10 includesliquefied nitrogen (LN₂) supply tank 20 that is usually located outsidethe building where the heat treating furnace 12 is installed. The supplytank 20 contains LN₂ at a pressure that is preferably greater than about2 bar. A first cryogenic pipe 31 connects the LN₂ supply tank 20 to anLN₂ storage tank 18 located in close proximity to the heat treatmentfurnace. A manual shut-off valve 42 is connected in the first cryogenicpipe 31, preferably in proximity to the supply tank 20. Asolenoid-operated control valve 44 is preferably connected in the firstcryogenic pipe 31 in proximity to the storage tank 18 for controllingthe flow of LN₂ to the storage tank 18. First and second vent valves 32a and 32 b are provided at respective first and second locations alongthe first cryogenic pipe 31. The first vent valve 32 a is preferablylocated closer to supply tank 20. The second vent valve 32 b ispreferably located closer to storage tank 18. The first and second ventvalves are typically embodied as spring-loaded safety relief devicesthat permit any overpressure in the cryogenic pipe 31 to be rapidlyreduced when the set pressure limit of the valve is exceeded by apressure buildup in the cryogenic pipe 31. The storage tank 18 isconstructed to handle cryogenic temperatures. Preferably the storagetank has a double-wall construction with a vacuum established in thespace between the inner and outer tank walls in order to minimize heattransfer into the storage tank 18. Alternatively or in addition, thestorage tank is thermally insulated to a degree necessary to maintainthe LN₂ at cryogenic temperature. A third vent valve 19 is provided onthe storage tank 18 to prevent over-pressurization of the storage tank.The first cryogenic pipe 31 may also be double-walled construction orhave sufficient thermal insulation to maintain the liquefied nitrogen ata cryogenic temperature.

The heat treating furnace 12 is constructed for holding a load of metalwork-pieces 16 that are heat treated in the furnace. The load willtypically be in the form of stacked baskets or containers of the metalwork pieces. The heat treating furnace 12 includes a pressure vessel orquenching chamber that is capable of holding a quenching gas, such asnitrogen, at pressures of at least about 5 bar up to about 100 bar. Thepressure vessel or quenching chamber preferably includes a recirculationfan 13 which operates to circulate the quenching gas in the furnacechamber. A heat exchanger (not shown) is also included for extractingheat from the quenching gas as it is recirculated through the heatexchanger. The heat exchanger is preferably located internally to thepressure vessel, but may be located externally in accordance witharrangements generally known to persons skilled in the art. Likewise,the recirculation fan may be located externally to the pressure vesselin accordance with arrangements generally known to persons skilled inthe art. One or more spray nozzles 15 a, 15 b, 15 c, may be connectedfrom a cryogenic manifold 14. A second cryogenic pipe 33 is connectedbetween the LN₂ storage tank 18 and the cryogenic manifold for supplyingLN₂ gas to the spray nozzles 15 a, 15 b, and 15 c. The LN₂ storage tankis preferably located in close proximity to the heat treating furnace,specifically to the quenching chamber of the furnace. In this way,second cryogenic pipe 33 is kept as short as possible. The secondcryogenic pipe 33 preferably has an inside diameter that is dimensionedto allow the LN₂ to flow into the manifold 14 at a rate of about 1 to 15l/s. Such a flow rate may allow the heat treating furnace 12 orquenching chamber to be pressurized to the desired quenching gaspressure within as little as 2-5 seconds. More typically, it is expectedthat the desired quenching gas pressure will be attained in about 10 toabout 50 or 60 seconds. The spray nozzles are preferably constructed toprovide a wide angle spray as shown in FIG. 2. A manual shut-off valve41 may be connected in the second cryogenic pipe 33 in proximity to thestorage tank 18. A solenoid-operated control valve 43 is connected inthe second cryogenic pipe 33 in proximity to the furnace 12 forcontrolling the flow of the LN₂ from the storage tank 18 to the manifold14 and the spray nozzles. A fourth vent valve 34, similar to vent valves32 a and 32 b is provided on the second cryogenic pipe 33 to preventover-pressurization of that line.

A pipe or tube 47 extends from the interior of the pressure vessel orquenching chamber 12 to provide an overpressure exhaust port. Asolenoid-operated valve 48 is connected in the pipe or tube 47 tocontrol the flow of quenching gas from the interior of the pressurevessel or quenching chamber through the exhaust port and out to theatmosphere when the gas pressure inside the pressure vessel reaches apredefined peak value.

A high pressure source of pressurizing gas 22, preferably nitrogen, isconnected to the storage tank 18 through high pressure gas tubing orpipe 35. The pressurizing gas source is preferably realized with a highpressure gas cylinder. A pressure regulator 26 may be connected in thehigh pressure tubing 35 in proximity to the high pressure gas source 22.A solenoid-operated control valve 46 is connected in the high pressuregas tubing 35 in proximity to the storage tank 18 for controlling theflow of gas from the source 22 to the storage tank 18. A pressure switch24 is provided at the heat treating furnace 12 and is adapted to sensethe gas pressure inside the pressure vessel or quenching chamber. Thepressure switch 24 is connected to the control valve 46 for controllingthe high pressure gas flow to the storage tank 18 from the gas source22. In an alternative embodiment, a cryogenic fluid pump (not shown) canbe connected in the LN₂ supply line 31 to pump the LN₂ up to a desiredpressure in the storage tank 18.

The filling of the storage tank 18 is achieved by establishing apositive pressure differential in the LN₂ supply tank 20 relative to thestorage tank 18. The volume of the storage tank 18 is selected such thatthe amount of LN₂ stored will be sufficient to bring the high pressuregas quench system of the heat treat furnace 12 to the desired gaspressure for quenching after evaporation of the liquefied nitrogen. Forexample, a high pressure gas quench system having a volume of 2 m³ canbe used for a quenching cycle that requires a gas pressure of 30 bar.This means that 60 m³ of nitrogen gas are needed to reach this pressure,which requires at least 90 liters of LN₂ to be filled into the LN₂storage tank 18.

When the storage tank 18 is filled with a sufficient amount of LN₂, itis closed-off completely by the valve 44 in the first cryogenic pipe 31and valve 43 in the second cryogenic pipe 33. The pressure inside thestorage tank is allowed to build up to a value sufficient to cause theliquefied nitrogen to flow from the storage tank 18 into the manifold 14and spray nozzles 15 a-15 c in the heat treat furnace 12 at a flow ratesufficient to provide an amount (volume) of LN₂ that will cause thedesired quench gas pressure to occur after evaporation of the LN₂ insidethe furnace.

To achieve rapid evaporation of the LN₂ inside the heat treating furnaceor quenching chamber, it is advantageous to spray the LN₂ flow with awidely diverging spray pattern. Although the embodiment shown in FIG. 2shows an arrangement of three spray nozzles, the preferred spray patterncan be provided by using only one or two nozzles so long as the nozzlesare constructed to provide a wide spray pattern.

Preferably, a constant pressure differential is maintained across thespray nozzles to provide a constant flow of LN₂. As an example of asuitable operating characteristic, the desired flow can be achieved byusing a starting pressure of about 5 bar in the storage tank 18 andincreasing the pressure in the storage tank during outflow of the LN₂ sothat the storage tank pressure is always higher than the instantaneousgas pressure in the pressure vessel by at least about 3 bar. Thus, afinal pressure of about 30 bar, for example, in the heat treatingfurnace 12 can be achieved by causing the pressure in the LN₂ storagetank to be about 33 bar, for example, during the cycle of supplying theliquefied nitrogen to the heat treating furnace. Alternatively, thepressure in the storage tank can be raised by starting at a pressure of5 bar and continuously raising it to about 33 or 35 bar during thefilling operation. The high pressure needed in the LN₂ storage tank iseasily established by connecting it to the source 22 of nitrogen gasunder very high pressure to the LN₂ storage tank.

The process according to the present invention is preferably realizedthrough use of the apparatus described above. However, it iscontemplated that other systems can be designed for carrying out theprocess. The quenching process according to the present invention ispreferably utilized in an industrial metal heat treating process. Such aprocess typically includes the steps of heating a load of metal workpieces in a heat treating furnace to a desired temperature and thenholding the metal work pieces at this temperature for a period of timesufficient to effect a desired metallurgical change in the metal workpieces. The heat treating furnace may be a vacuum furnace or anatmosphere furnace. The desired change in the metal work pieces is ofteneffected or locked in by cooling the metal work pieces at a rapid rate.

In the method according to the present invention the heated metal partsare cooled by application of a cooling gas, preferably nitrogen, at highpressure. The cooling gas is preferably injected into the furnace orquenching chamber by conducting LN₂ from a local storage tank into theheat treating furnace chamber or into a standalone quenching chamber asthe case may be. Feeding the LN₂ into a furnace quench chamber at a highflow rate against a gas pressure that has built up to about 25 bar ormore requires a pressure in the LN₂ storage tank of at least about 30bar or more. However, at such a pressure the boiling point of the LN₂rises to about −151° C., which is 45° C. higher than when the pressurein the storage tank is at 1 bar. The spraying of LN₂ at a temperature of−151° C. into the high pressure quench chamber results in a reduction ofthe cooling capability of the quenching medium by about 22% as comparedto spraying the LN₂ at a temperature of −196° C. Therefore, moreeffective cooling with LN₂ spray quenching can be provided when the LN₂is super-cooled. Super cooling of the LN₂ can be accomplished by usingthe following steps.

Prior to the injection of LN₂ into the heat treating furnace orquenching chamber, the LN₂ is preferably held in the storage tank 18 ata relatively low pressure, for example at about 1 bar. As the processproceeds and LN₂ flows toward the heat treating furnace or quenchingchamber, the pressure in the storage tank 18 is increased to a pressurethat is greater than the final pressure required for the specific gasquench cycle. Alternatively, the pressure in the LN₂ storage tank can beset directly to a pressure of at least about 3 bar at the start of thequenching cycle and then, while the LN₂ flows toward the furnace orquench chamber, the pressure in the LN₂ storage tank is continuouslyincreased at such a rate that the pressure is at any point of timeduring the quenching cycle at least 3 bar higher than the pressure inthe furnace or quench chamber at the same time. The pressure in thestorage tank is preferably increased or maintained, as the case may be,by injecting nitrogen gas at elevated pressure into the storage tank.The gas injection is preferably carried out by allowing nitrogen gasfrom the high pressure gas source 22 to flow into the storage tank 18thereby providing a blanket of gas whose pressure is determined by thepressure regulator 26.

It is understood, that in carrying out the process of this invention,the LN₂ will initially evaporate as it is conducted from the storagetank to the furnace or quenching chamber because the supply pipe fromthe storage tank to the furnace chamber will not initially be atcryogenic temperature. As the supply pipe cools down to cryogenictemperature, the nitrogen will enter the chamber as a combination ofcold nitrogen gas and liquefied nitrogen. When the supply pipe hascooled to substantially cryogenic temperature, the LN₂ will be conductedinto the spray manifold in the furnace chamber and exit from the spraynozzles to be sprayed over the batches of metal work pieces. Theconduction of the cooling gas in liquid form will provide a greater massof the cooling gas into the furnace chamber thereby causing the gaspressure in the furnace chamber to rise rapidly. More specifically, itis expected that peak gas pressure for cooling in the furnace chambercan be achieved in 30 seconds or less from the start of the liquefiedgas injection process.

During the injection of the cooling liquid into the furnace chamber, thevaporized nitrogen gas is preferably continuously circulated inside thechamber by means of the recirculation fan 13. The continuous circulationof the LN₂ mist and the cold nitrogen gas causes the gas/mist mixture topenetrate into the lower layers of the work piece load so that the lowerlayers of the stacked baskets or containers are cooled at the same or asimilar rate as the uppermost baskets of work pieces. As the nitrogengas/mist mixture absorbs heat from the metal work pieces, it transformsto all gas and rapidly expands inside the pressure vessel. The rapidexpansion of the gas causes the pressure to rapidly rise also.

Once the gas pressure inside the furnace chamber reaches the desiredpeak value, the injection of the LN₂ can be stopped. The recirculationfan preferably continues to run so that the quenching gas isrecirculated through the heat exchanger to remove additional heat fromthe load in the furnace chamber. The gas recirculation at the elevatedpressure continues until the work pieces reach a preselected temperaturein accordance with the known gas quenching processes.

Depending on the geometry of the load of metal parts, it may beadvantageous to spray the liquid quenchant in a particular direction tomaximize penetration of the gas/mist mixture into the work load. Whensuch directional spraying is used, it may also be preferable tocirculate the gas/mist mixture in a direction selected to furtherenhance contact of the cooling gas and mist with the metal parts.Therefore, in some embodiments the direction of circulation is selectedto be parallel to the spraying direction. In another embodiment, thecirculation of the gas and mist is circulated in a direction that is atan angle to the spraying direction, for example, at an angle of 90degrees or 180 degrees relative to the spraying direction.

Referring now to FIG. 3, there is shown an example of a first or lowpressure cooling cycle according to the present invention. In a firststage (1) of the cooling cycle, LN₂ is injected into a furnace chambercontaining a load of metal parts that is at an elevated heat treatmenttemperature. As the LN₂ is injected, the gas pressure builds up to apeak level of about 10 bar. This stage lasts for about 15 seconds afterwhich a first temperature (T1) is reached that is lower than theelevated heat treatment temperature. The gas recirculation fan is runsimultaneously with the injection of the liquefied gas. In a secondstage (2) the supply of LN₂ is stopped, but the gas pressure ismaintained at its peak level and the gas recirculation fan continues torun until a second temperature (T2) lower than the first temperature isreached. In a third stage (3), after temperature T2 is reached, the gaspressure is reduced to about 5 bar while the gas recirculation fan isstill running The third stage is continued until the work load reaches adesired third temperature (T3) that is lower than temperature T2. Forexample, T3 may be room temperature or a higher temperature.

Depending on the overall load size, the section size of the parts in theload, and especially the type of steel or metal of the parts, thequenching speed of the second stage in the process of this invention(i.e., circulation of gas at high pressure) might not be sufficient. Insuch situation, it is possible to further supply the liquid quenchantinto the furnace during (and vent off the vapor produced once itsupersedes the chosen final peak pressure) for an additional time periodduring the first stage, until subsequently the transition to the secondstage (pure high pressure gas quench) is made (stopping the flow ofliquid). Such a process is exemplified in the following description ofthe example illustrated in FIG. 4.

Referring now to FIG. 4, there is shown an example of a second or highpressure cooling cycle according to the present invention. In a firststage (1) of the second cooling cycle, LN₂ is injected into the furnacechamber containing a load of metal parts that is at an elevated heattreatment temperature. As the LN₂ is injected, the gas pressure buildsup to a peak level of about 25 bar. The peak pressure is reached inabout 20 seconds and the injection of LN₂ continues for an additionalperiod of time until a first temperature T1 is reached that is lowerthan the elevated heat treatment temperature. The peak pressure ismaintained by causing some of the cooling gas to be exhausted from thefurnace chamber through the exhaust pipe 47. This first stage lasts forup to about 30 seconds in this example. The gas recirculation fan is runsimultaneously with the injection of the liquefied gas. In a secondstage (2) the supply of LN₂ is stopped, the gas pressure is maintainedat its peak level, and the gas recirculation fan continues to run untila second temperature (T2) lower than the temperature T1 is reached. In athird stage (3), the gas pressure is reduced to about 5 bar while thegas recirculation fan is still running The third stage is continueduntil the work load reaches the desired third temperature T3 that islower than temperature T2.

During further cooling in the third stage of the process according tothis invention, i.e., pure gas quenching, the gas temperature decreaseswhich causes the gas to contract, thereby reducing the pressure in thequenching chamber. In order to maintain the pressure during a givencooling stage constant, the pressure control system is preferablyadapted to intermittently open the valve for the liquid quenchant andallow more liquid to enter the furnace. The evaporation of theadditional liquid increases the pressure in the quenching chamber backto the desired level.

It will be appreciated by those skilled in the art that the apparatusaccording to the invention can be realized by configurations other thanthat described above and shown in FIG. 2. It is contemplated by theinventors that the process according to the present invention can becarried out in any of numerous quenching cycle sequences. Thus, theinvention is not limited to the two examples described above and shownin FIGS. 3 and 4. Moreover, the process and apparatus according to theinvention can be used with a wide variety of liquid quenchants otherthan LN₂. Thus, it is believed that the process can be conducted withsuch other quenchants as liquefied helium, liquefied argon, liquefiedair, a liquefied hydrocarbon, liquefied carbon dioxide, and acombination thereof. Moreover, the process according to the inventioncan be carried as a high pressure steam quench utilizing a liquidquenchant such as water, an aqueous quenchant solution, or a quenchingoil. Quenchant solutions and quenching oils are well known to thoseskilled in the art as well as the knowledge of how to select a suitableoil or quenchant solution given the load size, part geometry, and partmaterial.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation. There is no intention in the use ofsuch terms and expressions of excluding any equivalents of the featuresor steps shown and described or portions thereof. It is recognized,therefore, that various modifications are possible within the scope andspirit of the invention. Accordingly, the invention incorporatesvariations that fall within the scope of the following claims.

1. A method for rapidly cooling a load of heat treated metal parts froman elevated temperature comprising the steps of: providing a load ofheat treated metal parts in a pressure vessel, said load being at anelevated temperature after being heat treated; injecting a liquidquenchant into the pressure vessel such that a vapor of the liquidquenchant forms rapidly in the pressure vessel and cools the metalparts, and then continuing to inject the liquid quenchant into thepressure vessel for a time sufficient to establish a desired peak vaporpressure in the pressure vessel.
 2. A method as claimed in claim 1wherein the desired peak vapor pressure is about 5 to 100 bar.
 3. Amethod as claimed in claim 1 comprising the step of circulating thequenchant vapor at high velocity in the pressure vessel while the liquidquenchant is being injected into the pressure vessel such that thequenchant vapor penetrates through the load of metal parts.
 4. A methodas claimed in claim 3 wherein the injecting step comprises spraying theliquid quenchant in a preselected direction in the pressure vessel.
 5. Amethod as claimed in claim 1 comprising the step of providing the liquidquenchant at an initial pressure that is higher than the desired peakvapor pressure in the pressure vessel.
 6. A method as claimed in claim 5wherein the liquid quenchant has an initial pressure that is higher thanthe quenchant vapor pressure in the pressure vessel by at least about 3bar.
 7. A method as claimed in claim 1 wherein the liquid quenchant hasan initial pressure that is about 3 to 5 bar.
 8. A method as claimed inclaim 1 wherein the injecting step comprises the step of continuouslyraising the pressure of the liquid quenchant during the injecting stepsuch that the pressure of the liquid quenchant at any instant is higherthan a concurrent quenchant vapor pressure in the pressure vessel.
 9. Amethod as claimed in claim 8 wherein the pressure of the liquidquenchant at any instant is about 3 to 5 bar higher than the concurrentvapor pressure in the pressure vessel.
 10. A method as claimed in claim1 wherein the injecting step is stopped once the desired peak vaporpressure in the pressure vessel is reached.
 11. A method as claimed inclaim 10 comprising the steps of maintaining the quenchant vaporpressure in the pressure vessel at the desired peak vapor pressure andcontinuing to circulate the quenchant vapor for a time sufficient tolower the temperature of the metal parts to a first temperature lowerthan the elevated temperature.
 12. A method as claimed in claim 1comprising the steps of continuing the injecting step and maintainingthe vapor pressure in the pressure vessel at the desired peak vaporpressure for a period of time after the desired peak vapor pressure inthe pressure vessel is reached sufficient to lower the temperature ofthe metal parts to a first temperature lower than the elevatedtemperature.
 13. A method as claimed in claim 12 wherein the peak vaporpressure in the pressure vessel is maintained at the desired level byventing a portion of the quenchant vapor from the pressure vessel.
 14. Amethod as claimed in claim 11 or 12 wherein the peak vapor pressure inthe pressure vessel is maintained at the desired pressure by injectingadditional quenchant vapor into the pressure vessel.
 15. A method asclaimed in claim 11 comprising the step of reducing the quenchant vaporpressure in the pressure vessel to a lower pressure when the load ofmetal parts reaches the first temperature.
 16. A method as claimed inclaim 15 comprising the step of holding the quenchant vapor pressure inthe pressure vessel at the lower pressure until the load of metal partsreaches a selected second temperature lower than the first temperature.17. A method as claimed in claim 3 wherein the circulating stepcomprises the step of circulating the quenchant vapor through a heatexchanger located in the pressure vessel and circulating a heatabsorbing fluid in the heat exchanger to absorb heat from the quenchantvapor.
 18. A method as claimed in claim 1 wherein the injecting step iscarried out with a flow rate that is effective to raise the vaporpressure in the pressure vessel to the desired peak vapor pressurewithin about 2 to 60 seconds from the start of the injecting step.
 19. Amethod as claimed in claim 1 wherein the liquid quenchant is selectedfrom the group consisting of liquefied nitrogen, liquefied helium,liquefied argon, liquefied air, a liquefied hydrocarbon gas, liquefiedcarbon dioxide, and a combination thereof
 20. A method as claimed inclaim 1 wherein the liquid quenchant is water, an aqueous quenchingsolution, or oil.
 21. Apparatus for rapidly cooling a work load of heattreated metal parts comprising: a pressure vessel having an internalchamber for holding a work load of heat treated metal parts; a liquidquenchant supply vessel adapted to contain a liquid quenchant at a firstpressure; quenchant conducting means for conducting the liquid quenchantfrom the supply vessel to the internal chamber of the pressure vessel;and pressure control means operatively connected to said pressure vesseland said quenchant conducting means for maintaining the liquid quenchantconducted to said pressure vessel at an elevated pressure differentialsufficient to establish a desired peak vapor pressure in the internalchamber of the pressure vessel.
 22. An apparatus as claimed in claim 21wherein said pressure control means is adapted for controlling the flowrate of the liquid quenchant from said supply vessel to the internalchamber of the pressure vessel.
 23. Apparatus as claimed in claim 21wherein the quenchant conducting means comprises a means for increasingthe pressure of the liquid quenchant conducted to the pressure vessel.24. Apparatus as claimed in claim 23 wherein the pressure increasingmeans comprises a liquid pump.
 25. Apparatus as claimed in claim 23wherein the pressure increasing means comprises a source of pressurizedgas.
 26. Apparatus as claimed in claim 22 wherein the quenchantconducting means comprises: a storage tank adapted for concurrentlyholding liquid and vapor phases of the quenchant; and means forincreasing pressure inside said storage tank.
 27. Apparatus as claimedin claim 26 wherein the pressure increasing means comprises a fluidpump.
 28. Apparatus as claimed in claim 26 wherein the pressureincreasing means comprises a source of pressurized gas.
 29. Apparatus asclaimed in claim 28 wherein the means for increasing the pressure in thestorage tank comprises a source of pressurizing gas at a second pressuregreater than said first pressure and means for conducting thepressurizing gas at said second pressure from said source to saidstorage tank.
 30. Apparatus as claimed in claim 21 comprising a nozzleadapted for spraying the liquid quenchant in the pressure vesselchamber, said nozzle being operably connected to said quenchantconducting means and mounted in the internal chamber of the pressurevessel.
 31. Apparatus as claimed in claim 30 wherein the pressure vesselis part of a heat treating furnace.
 32. Apparatus as claimed in claim 30wherein the pressure vessel is a standalone quenching chamber. 33.Apparatus as claimed in claim 21 comprising a fan operatively coupled tosaid pressure vessel for circulating quenchant vapor in the internalchamber of said pressure vessel.
 34. Apparatus as claimed in claim 33comprising a heat exchanger connected to said pressure vessel forextracting heat from the quenchant vapor as it is circulated in thepressure vessel.
 35. Apparatus as claimed in claim 28 wherein the meansfor conducting the pressurizing gas comprises a pressure regulatoroperably connected to the pressurizing gas source.
 36. Apparatus asclaimed in claim 30 comprising a second nozzle for spraying the liquidquenchant, said second nozzle being mounted in the pressure vessel andoperatively connected to the liquid quenchant conducting means. 37.Apparatus as claimed in claim 30 wherein the quenchant conducting meanscomprises a manifold in the internal chamber of the pressure vessel andthe nozzle is connected to said manifold.
 38. Apparatus as claimed inclaim 37 comprising a second nozzle connected to said manifold.